The present invention relates to electric heater controls and more specifically to means for modulating the duty cycle of glow plugs used for starting diesel engines.
In order to start a diesel engine, such as one of the prechamber injection type, it is common practice to employ electrically heated glow plugs which extend into the chamber and to preheat these plugs to a temperature above the flash temperature of the fuel prior to cranking of the engine, known as a preglow stage, and thereafter to maintain the plugs at such temperature for a selected period of time while the engine warms up sufficiently, known as an after glow stage, and then to deenergize the plugs. Historically the preglow stage has been considered excessively long for people accustomed to conventional spark ignition engines since typically a full minute or more of glow plug preheating was required before the engine could be cranked. Such glow plugs in effect are high resistance heater elements and are heated by current drawn from the battery until the glow plugs reach approximately 1800.degree. F. when a so called "wait" lamp is deactivated indicating that the engine is ready to be cranked. After a selected time delay to provide an after glow period the glow plugs are deenergized. Such glow plugs not only require excessive time for preheating but the time required has been too dependent upon the level of the supply voltage. That is, if the voltage level happened to be somewhat low significantly longer time was required to preheat the glow plugs until they reached the flash temperature of the fuel.
Recently, a fast start system has been developed in which the glow plugs are heated to the desired temperature much more quickly, in the order of six or seven seconds. This is accomplished by using glow plugs with a lower resistance to obtain more heat generation and to thereafter cycle the glow plugs on and off with a selected duty cycle in order to maintain the glow plugs within a selected range for the after glow period. Such a system is described in a technical paper published by the SAE (Society of Automotive Engineers) on Feb. 26-Mar. 2, 1979, entitled "Design of a Fast Start Glow Plug Control System For Diesel Engines" written by Arthur R. Sundeen. While the control described in the paper is effecetive in preheating the glow plugs in a greatly reduced time period and is effective in maintaining the plugs within a selected temperature range for an afterglow period it does suffer from several disadvantages. That is, the control employs three creep acting bimetallic switches arranged to operate in a particular sequence. Each bimetal is heated by a wire wound or printed heater. There are many hand operations required in making the control, e.g. soldering of the wire or painted heaters to associated parts, calibration of the switches to assure the desired sequential operation, and the like. As a result the control is inherently expensive to produce and subject to problems of yield and reliability. For instance the heater wire wound about the bimetallic blade is of small gauge and could easily be broken thereby changing the sequence of operation.